Data will be obtained to evaluate the relative potency of two enantiomers of methylphenidate (MPH) on activity and complex information processing of rodents (rats). As MPH is a widely prescribed psychostimulant for hyperactivity and attention deficit disorders in children, we must carefully evaluate the relative potency of alternate forms of the drug to minimize side-effects and improve therapeutic effectiveness. Recent development of a method to extract d-MPH and l-MPH from biological samples makes it possible to produce a purified compound. We will evaluate the relative potency of these two isomers on three measures of rat behavior including locomotor activity, precision of time estimation, and discrimination of relative reinforcement rates. These measures were selected as relevant to effects of MPH found in the clinical literature. Dose-response functions for MPH and each isomer will be obtained.